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          vue之条件渲染
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        <p>click look more</p>
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<h1 id="v-if"><a href="#v-if" class="headerlink" title="v-if"></a>v-if</h1><blockquote>
<p>v-if的作用是字面意思，即条件为真则显示，反之不显示</p>
</blockquote>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">h1</span> <span class="attr">v-if</span>=<span class="string">"awesome"</span>&gt;</span>Vue is awesome!<span class="tag">&lt;/<span class="name">h1</span>&gt;</span></span><br></pre></td></tr></table></figure>

<p>假设这里awesome为false，那么h1就不会显示，如果加个else就不一样了，反之逻辑就是这样！</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">h1</span> <span class="attr">v-if</span>=<span class="string">"awesome"</span>&gt;</span>Vue is awesome!<span class="tag">&lt;/<span class="name">h1</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">h1</span> <span class="attr">v-else</span>&gt;</span>Oh no 😢<span class="tag">&lt;/<span class="name">h1</span>&gt;</span></span><br></pre></td></tr></table></figure>

<h2 id="在模板（template）元素上使用-v-if-条件渲染分组"><a href="#在模板（template）元素上使用-v-if-条件渲染分组" class="headerlink" title="在模板（template）元素上使用 v-if 条件渲染分组"></a>在模板（template）元素上使用 v-if 条件渲染分组</h2><blockquote>
<p>因为v-if是一个指令，所以必须用在元素上。那就说明只能控制一个元素的显示与否。事实果真如此吗？</p>
</blockquote>
<p>其实只需要在外层包裹一个不可见的template元素即可</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">template</span> <span class="attr">v-if</span>=<span class="string">"ok"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">h1</span>&gt;</span>Title<span class="tag">&lt;/<span class="name">h1</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">p</span>&gt;</span>Paragraph 1<span class="tag">&lt;/<span class="name">p</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">p</span>&gt;</span>Paragraph 2<span class="tag">&lt;/<span class="name">p</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">template</span>&gt;</span></span><br></pre></td></tr></table></figure>

<h2 id="v-else"><a href="#v-else" class="headerlink" title="v-else"></a>v-else</h2><blockquote>
<p>这里上面已经使用过，这里说一点，即<strong>v-else必须跟在v-if 或者 v-else-if 后面，否则不会生效</strong></p>
</blockquote>
<h2 id="v-else-if"><a href="#v-else-if" class="headerlink" title="v-else-if"></a>v-else-if</h2><blockquote>
<p>同 v-else 一样。 不在过多赘述</p>
</blockquote>
<h2 id="用key管理可复用的元素"><a href="#用key管理可复用的元素" class="headerlink" title="用key管理可复用的元素"></a>用key管理可复用的元素</h2><blockquote>
<p>顾名思义，当添加key值时，被添加的元素不会被复用，反之则会被服用。看下面代码理解</p>
</blockquote>
<p>不使用key，你所填写的东西也会被服用</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">template</span> <span class="attr">v-if</span>=<span class="string">"loginType === 'username'"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">label</span>&gt;</span>Username<span class="tag">&lt;/<span class="name">label</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">input</span> <span class="attr">placeholder</span>=<span class="string">"Enter your username"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">template</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">template</span> <span class="attr">v-else</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">label</span>&gt;</span>Email<span class="tag">&lt;/<span class="name">label</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">input</span> <span class="attr">placeholder</span>=<span class="string">"Enter your email address"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">template</span>&gt;</span></span><br></pre></td></tr></table></figure>

<p>使用key，所填写的东西不会被服用，但是要牢记，例如 label ，它并没有key值，所以还是会被复用。实际上vue很快的原因有一部分在于此。</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">template</span> <span class="attr">v-if</span>=<span class="string">"loginType === 'username'"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">label</span>&gt;</span>Username<span class="tag">&lt;/<span class="name">label</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">input</span> <span class="attr">placeholder</span>=<span class="string">"Enter your username"</span> <span class="attr">key</span>=<span class="string">"username-input"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">template</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">template</span> <span class="attr">v-else</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">label</span>&gt;</span>Email<span class="tag">&lt;/<span class="name">label</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">input</span> <span class="attr">placeholder</span>=<span class="string">"Enter your email address"</span> <span class="attr">key</span>=<span class="string">"email-input"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">template</span>&gt;</span></span><br></pre></td></tr></table></figure>

<h1 id="v-show"><a href="#v-show" class="headerlink" title="v-show"></a>v-show</h1><blockquote>
<p>v-show 用法和 v-if大致相当，但是有几点不同。<strong>一个是v-show控制元素显示与否的方法是给元素添加了一个css属性，display none 。  v-if是直接消灭掉别隐藏的dom ；另一个是 v-show 没有 v-if的逻辑块；最后一个是没有template，即不可以同时控制多个元素</strong></p>
</blockquote>
<h1 id="v-if-VS-v-show"><a href="#v-if-VS-v-show" class="headerlink" title="v-if VS v-show"></a>v-if VS v-show</h1><blockquote>
<p>在介绍了v-show的基本原理后就会有一个问题，即何时使用v-if或者v-show？</p>
</blockquote>
<p>总结一下：</p>
<ul>
<li>v-if 在页面初始时会根据条件值去判断是否该去渲染，所以这会造成<strong>切换</strong>时的开销</li>
<li>v-show 在页面初始时无论如何都会渲染，所以这会造成<strong>启动</strong>页面时的开销。</li>
<li>在一个组件或者元素需要被频繁切换（即改变条件，下同）时，就使用 v-show 。因为它只需要改变css即可</li>
<li>在一个组件或者元素很少被切换时，就使用v-if</li>
</ul>
<h1 id="v-if-和-v-for一起使用"><a href="#v-if-和-v-for一起使用" class="headerlink" title="v-if 和 v-for一起使用"></a>v-if 和 v-for一起使用</h1><blockquote>
<p><strong>不推荐使用。</strong>因为 v-for 的优先级比 v-if 更高</p>
</blockquote>

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          <div class="post-toc motion-element"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#v-if"><span class="nav-number">1.</span> <span class="nav-text">v-if</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#在模板（template）元素上使用-v-if-条件渲染分组"><span class="nav-number">1.1.</span> <span class="nav-text">在模板（template）元素上使用 v-if 条件渲染分组</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#v-else"><span class="nav-number">1.2.</span> <span class="nav-text">v-else</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#v-else-if"><span class="nav-number">1.3.</span> <span class="nav-text">v-else-if</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#用key管理可复用的元素"><span class="nav-number">1.4.</span> <span class="nav-text">用key管理可复用的元素</span></a></li></ol></li><li class="nav-item nav-level-1"><a class="nav-link" href="#v-show"><span class="nav-number">2.</span> <span class="nav-text">v-show</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#v-if-VS-v-show"><span class="nav-number">3.</span> <span class="nav-text">v-if VS v-show</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#v-if-和-v-for一起使用"><span class="nav-number">4.</span> <span class="nav-text">v-if 和 v-for一起使用</span></a></li></ol></div>
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